What makes someone "boring?"?

In a group it will depend on what is general trend they’re following. Depending on that they’ll label you as boring if you’re not with them. In reality a boring person in my opinion is one who can’t find happiness in small moments, who can’t enjoy beauty of nature and tries to find loophole in everything he sees. That I would call boring.

Not being able to carry on a conversation.

Generally, “You’re so boring” is a speech made by one lacking the imagination, intelligence or sense to appreciate the qualities of the target of the speech. In short, a reflection of the speaker’s own qualities onto the wrong person.

A person who’s boring to me is someone who can only talk about people (including himself or herself) or, heaven help us, their pets, and not ideas or the larger world. The person has no independent ideas, no creative imagination, no ability to transform experience or sensory input into thought. They can talk on and on, but it isn’t conversation.

I imagine that I could be boring to people who do want to talk about celebrities, babies, and pets. That’s fine with me.

Not that there’s anything wrong with celebrities, babies, and pets, but thought shouldn’t stop there.

People who can’t participate in a meaningful interchange. Their whole conversation revolves around them (or their limited interests). A conversation should involve at least two voices. I also find people who say ‘I’m bored’ or ‘this is boring’ boring. I’m very rarely bored and, with the exception of those who hog the conversation or have very narrow interests, most people have something interesting to say if you can encourage them to share their ideas and thoughts.

^^Funny what you said about people who say, “I’m bored.” It’s true that often those people are boring.

I don’t always find people who talk about themselves or their interests boring. It really depends on the topic. I guess there needs to be a little give and take, but I could listen to a long stretch if the information or story they are telling is funny or knew knowledge I’m interested in.

That’s why I said “their whole” conversation. Some people seem to have just one note. They can only talk about their very narrow interest(s). My work is about getting people to talk about themselves, and I find it totally fascinating too. I enjoy listening to people talk about their passions, but usually those people can discuss their passion within a wider context too. And then there are people who seem to have really narrow interests. Those who can only talk about their local football team, or trainspotting or the latest diet they are on. That becomes boring fast.

^^Agreed.

Repetitivness and monotone speaking, especially if the person talks about something you are obviously not interested in.
Whining and nagging can make one boring as well.

LOL. The details to your question leave me absolutiely baffled.

A person who talks non-stop and can’t enjoy a comfortable moment of silence.

Boredom is about the bored person, not about what they are blaming for their boredom.

To me, when someone talks incessantly about themselves, they’re boring. If they can’t bring the conversation out of their own little world, they’re boring.

We’re the ones who dictates what is boring for us. I would never blame other people if they make me feel bored, the problem lies in whether or not they fit in the standard I’ve set in having the relationship with them. I could get bored easily with a nice person who like to help anyone all the time and it’s just simply because of my personal taste as I prefer someone with both dark and bright side to keep our relationship engaging. It’s not a simple matter of repetition and monotonous habit.

It’s entirely subjective. Someone becomes boring as soon as we have had enough of their company.

Someone who can’t get off a subject after it has been beaten to death, or insists on vehemently spewing some extreme left or right wing political opinion, or can’t get enough of the sound of their own voice.

Predictable
Repetitive
Monotonous

I was with some friends the other night and another guest was this woman who was very nice, but the conversation from her was her cats, her house, what she ate for Thanksgiving, her job, her car. I was like this woman is nice but she has the personality of a dish rag.

I tend to agree with @Jeruba and @Espiritus_Corvus

I enjoy discussing interesting things and while I don’t always need to be intellectualizing I do find those that have few interests or an abysmal lack of curiosity to be boring. Recently I was all excited about the spotting of several Jaguars in AZ. and most recently, several female Ocelots , one with kittens, spotted in Texas. I get really excited about the natural world and especially sightings of threatened or previously not seen species of animals returning to their former ranges. I know I was boring the person I was talking to as they are not a very enthusiastic or curious type.

They are one that prefers to talk mostly about the mundane details of their life and other people. Incredibly “boring” for me. I don’t care to hear all about your grocery shopping trip or what day you wash your hair. Gah! haha

When a person spends too much time talking about a single subject. Someone who goes on and on and on and the listener’s eyes may glaze over but the person still rambles on.